1. Field of the Invention
The present invention relates to a transmission device and a control method for a continuously variable transmission. More specifically, the present invention relates to a transmission device including a continuously variable transmission that is installed in a vehicle and that is shifted up by supplying a working fluid to a working fluid chamber and that is shifted down by discharging the working fluid from the working fluid chamber. The present invention also relates to a control method for such a continuously variable transmission.
2. Description of the Related Art
As an example of this type of transmission device, a transmission device including a belt type continuously variable transmission that is installed in a vehicle and driven by hydraulic pressure has been proposed. In this known transmission device, when the vehicle is not running at a low-speed that is equal to or lower than a predetermined vehicle speed, feedback control is performed on the gear ratio of the continuously variable transmission based on a deviation between the actual number of revolutions of a predetermined revolving member and a target revolution number, whereas when the vehicle is running at a low speed, the feedback control is inhibited and also closing control is performed on the hydraulic circuit so that the gear ratio of the continuously variable transmission does not change (see, for example, Japanese Patent Application Publication No. JP-A-2001-324007).
In the transmission device described above, when the vehicle is running at a low-speed, the closing control is performed so that the gear ratio of the continuously variable transmission does not change. However, over the course of time, there is a possibility that the gear ratio of the continuously variable transmission may gradually change because oil seeps through a small gap in a valve in the hydraulic circuit. In the case where, for example, the gear ratio of the continuously variable transmission has changed in the upshift direction, because the reduction gear ratio has become smaller, even if the driver depresses the accelerator pedal, only a small amount of driving force is output. Thus, there is a possibility that acceleration performance when the vehicle starts moving may be degraded.